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United States Patent |
5,018,852
|
Cheng
,   et al.
|
May 28, 1991
|
Motion detection, novelty filtering, and target tracking using an
interferometric technique with GaAs phase conjugate mirror
Abstract
A method and apparatus for detecting and tracking moving objects in a noise
environment cluttered with fast- and slow-moving objects and other
time-varying background. A pair of phase conjugate light beams carrying
the same spatial information commonly cancel each other out through an
image subtraction process in a phase conjugate interferometer, wherein
gratings are formed in a fast photorefractive phase conjugate mirror
material. In the steady state, there is no output. When the optical path
of one of the two phase conjugate beams is suddenly changed, the return
beam loses its phase conjugate nature and the interferometer is out of
balance, resulting in an observable output. The observable output lasts
until the phase conjugate nature of the beam has recovered. The observable
time of the output signal is roughly equal to the formation time of the
grating. If the optical path changing time is slower than the formation
time, the change of optical path becomes unobservable, because the index
grating can follow the change. Thus, objects traveling at speeds which
result in a path changing time which is slower than the formation time are
not observable and do not clutter the output image view.
Inventors:
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Cheng; Li-Jen (LaCrescenta, CA);
Liu; Tsuen-Hsi (Northridge, CA)
|
Assignee:
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The United States of America as represented by the Administrator of the (Washington, DC)
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Appl. No.:
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568129 |
Filed:
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August 16, 1990 |
Current U.S. Class: |
356/28.5; 250/201.9; 356/450; 356/457 |
Intern'l Class: |
G01P 003/36; G01B 009/02; G01B 009/21 |
Field of Search: |
356/28.5,345,347
250/201.9
|
References Cited
U.S. Patent Documents
3572882 | Mar., 1971 | Neumann.
| |
4215936 | Aug., 1980 | Winocur.
| |
4571080 | Feb., 1986 | Papuchon et al.
| |
4575245 | Mar., 1986 | Borde.
| |
4659223 | Apr., 1987 | Huignard et al.
| |
4767195 | Aug., 1988 | Pepper.
| |
4921353 | May., 1990 | Chiou et al. | 356/347.
|
Other References
Fischer, Baruch et al., "New Optical Gyroscope Based on the Ring Passive
Phase Conjugator", Appl. Phys. Lett. 47 (1), Jul. 1985.
|
Primary Examiner: Buczinski; Stephen C.
Attorney, Agent or Firm: Jones; Thomas H., Adams; Harold W., Manning; John R.
Goverment Interests
ORIGIN OF THE INVENTION
The invention described herein was made in the performance of work under a
NASA contract, and is subject to the provisions of Public Law 96-517 (35
U.S.C. Section 202) in which the Contractor has elected not to retain
title.
Claims
What is claimed is:
1. Apparatus for detecting an object moving in excess of a selected speed
comprising:
means for generating an image signal from an object scene;
an interferometer means comprising:
a photorefractive crystal means;
a means for generating first and second pump beams and first and second
source beams for forming first and second gratings in said photorefractive
crystal means;
said interferometer means further generating first and second phase
conjugate beams by diffraction of said second pump beam by said first and
second gratings, the first phase conjugate beam being 180 degrees out of
phase with the second;
said interferometer means further operating to combine said first and
second phase conjugate beams for forming an output beam comprising an
interference pattern which is destructive in the absence of an object
moving in excess of said selected speed and nondestructive when an object
moving above said selected speed is present in said object scene;
spatial light phase modulator means for spatially modulating the phase
front of said first source beam in accordance with said image signal; and
means interacting with said output beam for providing an output image
signal when an object moving above said selected speed is present in said
object scene.
2. The apparatus according to claim 1 further including a sample-and-hold
circuit means coupled between said image signal generating means and said
spatial light phase modulator means for sampling said input signal and for
holding the sampled input signal for a predetermined amount of time.
3. The apparatus of claim 2 wherein the sample-and-hold circuit means
comprises means for storing a matrix of electrical signals corresponding
to a selected frame of said image signal.
4. The apparatus according to claim 1 wherein said generating means
comprises a high speed movie camera means.
5. The apparatus according to claim 1 wherein said spatial light phase
modulator comprises:
a transparent ground electrode layer;
a layer of transparent electro-optical material disposed on top of said
ground electrode; and
a matrix of transparent electrodes disposed on top of said layer.
6. The apparatus according to claim 1 wherein said means for generating
said first and second source beams includes:
a laser; and
beam splitter means for generating said first and second source beams.
7. The apparatus for detecting and tracking fast- and slow-moving objects
according to claim 6 wherein said laser source exhibits a wavelength of
substantially 1.06 microns.
8. The apparatus of claim 1 wherein said fast photorefractive crystal means
comprises a gallium arsenide crystal.
9. The apparatus of claim 1 wherein said fast photorefractive crystal means
comprises an indium phosphide crystal.
10. The apparatus of claim 1 wherein said fast photorefractive crystal
means comprises cadmium telluride crystal.
11. The apparatus of claim 1 wherein said first photorefractive crystal
means comprises a gallium phosphide crystal.
12. An apparatus for detecting and tracking objects in a noise environment
cluttered with other time-varying background, comprising:
an input means for providing an input signal;
a light modulation means coupled to said input means;
a light beam means for emitting a light beam;
a beam splitting means optically coupled to said light beam means for
forming first and second source light beams, a first pump light beam and a
second pump light beam, said first source light beam transmitted through
said light modulation means for modulation thereof;
reflection means for directing said first and second source beams along
first and second separate predetermined converging optical paths and for
directing said second pump beam along a third path;
photorefractive means disposed in said converging optical paths and having
an incident surface for forming first and second gratings therein in
response to the incidence of said first and second source light beams and
said first pump beam, said photorefractive means having a second surface
on a side disposed in said third path, said photorefractive means thereby
receiving said second pump beam through said second surface;
said second pump beam interacting with said gratings to form first and
second phase conjugate light beams transmitted along the respective
optical paths of said first and second source light beams in a direction
opposite thereto;
said beam splitting means forming an output beam in response to said first
and second phase conjugate light beams; and
an output means for displaying an image present in said output beam;
whereby when the optical path of one of said phase conjugate light beams is
changed at a selected rate, the other of said phase conjugate light beams
loses its phase conjugate properties, thus resulting in display of an
image by said output means.
13. The apparatus according to claim 12 further including a sample-and-hold
circuit means coupled between said input means and said light phase
modulation means for sampling said input signal and for holding the
sampled input signal for a predetermined amount of time.
14. The apparatus of claim 13 wherein the sample-and-hold circuit means
comprises means for storing a matrix of electrical signals corresponding
to a selected frame of an image signal.
15. The apparatus according to claim 12 wherein said input means comprises
a high speed movie camera means.
16. The apparatus according to claim 12 wherein said light modulation means
comprises:
a transparent ground electrode layer;
a layer of transparent electro-optical material disposed on top of said
ground electrode; and
a matrix of transparent electrodes disposed on top of said layer.
17. A method of image detection utilizing a phase conjugate interferometer
having an output comprising the combination of first and second phase
conjugate beams, said method comprising the steps of:
spatially modulating a light beam in one arm of said phase conjugate
interferometer in accordance with the image in an object scene;
generating said first and second phase conjugate beams in said
interferometer by passing a coherent pump beam through gratings formed in
a fast photorefractive crystal; and
supplying said output to a means for converting said output into a visual
display, whereby said visual display displays an object in said object
scene which is moving at a speed above a speed selected at least in part
by the grating formation time of said crystal.
Description
TECHNICAL FIELD
The subject invention relates generally to motion detection and, more
particularly, to an apparatus for detecting and tracking fast- and
slow-moving objects in a noise environment, cluttered with other
time-varying background.
BACKGROUND ART
Prior art tracking novelty filters have been employed at least since the
early days of radar, when they were used to keep radar screens from
becoming cluttered by nonmoving objects. A tracking novelty filter is
readily implemented using a digital computer to subtract incoming images,
pixel by pixel, from a stored reference image that is periodically
updated. In 1987, Anderson, Lininger, and Feinberg proposed the concept of
constructing a tracking novelty filter using a Michelson interferometer
with a phase conjugate mirror. Such optical systems have the advantage of
fast operation because of the parallelism of light.
Previous publications and demonstrations of such systems have involved the
use of an interferometric configuration including a self-pumped phase
conjugate mirror formed of barium titanate. This approach exhibits a slow
response time and is therefore only suitable for detecting moving objects
in a steady background. This approach is not sufficient for detecting fast
(or accelerating) moving objects in a more realistic noise environment
cluttered with other time-varying background.
STATEMENT OF THE INVENTION
It is therefore an object of the invention to improve motion detection
systems;
It is another object of the invention to provide improved apparatus for
detecting and tracking fast- and slow-moving objects in a noise
environment cluttered with other time-varying background;
It is another object of the invention to provide an improved motion
detection system employing an optical system;
It is another object of the invention to provide an improved motion
detecting system employing an interferometer and phase conjugate mirror;
and
It is yet another object of the invention to improve the response time of
optical motion detecting systems and, in particular, such a system
employing an interferometer and phase conjugate mirror.
These and other objects and advantages are achieved according to the
invention by using fast photorefractive crystals such as gallium arsenide
(GaAs), indium phosphide (Inp), gallium phosphide (GaP), or cadmium
telluride (CdTe) as the high speed dynamic holographic recording medium of
a phase conjugate interferometer employed in a target detection and
tracking system. An object scene is introduced to the phase conjugate
interferometer by a spatial phase modulator which modulates the light in
one of two optical paths of the interferometer. The spatial light phase
modulator may be controlled to modulate the light in accordance with the
output of a sample hold circuit.
The minimum detectable speed (MDS) of the system is approximately the
spatial resolution of the system divided by the grating formation time of
the fast photorefractive crystal. Any objects moving at speeds slower than
the MDS are not detected. The MDS value of a system can be adjusted by
changing the grating periodicity or light intensity in the system. The MDS
can be further controlled by the sample-and-hold circuit, as discussed
hereafter.
The invention thus provides a thresholding technique for detecting and
tracking only the objects moving at speeds over a certain value.
Therefore, the system can detect and track fast- and slow-moving objects
in a noise environment cluttered with other time-varying background.
BRIEF DESCRIPTION OF THE DRAWINGS
The just-summarized invention will now be described in detail in
conjunction with the drawings, of which:
FIG. 1 is a schematic diagram of the preferred embodiment;
FIG. 2 is a perspective schematic view of a portion of a spatial light
phase modulator (SPM) according to the preferred embodiment;
FIGS. 3 and 4 are perspective schematic views of an individual SPM element,
respectively, with and without an applied electric field;
FIG. 5 is a graph illustrative of operating principles of the invention;
and
FIG. 6 is a schematic diagram illustrative of expected observations in a
system according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
In an optical approach employed according to the preferred embodiment, a
pair of phase conjugate beams carrying the same spatial information
(intensity and phase) commonly cancel each other out, through an image
subtraction process in a phase conjugate interferometer. Therefore, in the
steady state, there is no output. When the optical path of one of the two
phase conjugate beams is suddenly changed, the return beam loses its phase
conjugate nature and the interferometer is out of balance. Consequently,
an observable output results. The observable output lasts until the index
grating of the phase conjugate mirror of the interferometer is altered to
adapt to the change; that is, until the phase conjugate nature of the beam
has recovered. The observable time of the output signal is roughly equal
to the formation time of the grating. This phenomenon provides a means for
detecting motion. If the optical path changing time is slower than the
formation time, the change is unobservable, because the index grating can
follow the change, thus the phase conjugate relationship is always
preserved. The minimum detectable rate of change of the optical path is
approximately equal to the formation time of the grating. This rate can be
varied in accordance with system parameters.
A target detection and tracking system 10 according to the preferred
embodiment and employing the foregoing approach is illustrated in FIG. 1
of the drawings. The system 10 of FIG. 1 includes a first high-speed movie
camera 11, which may comprise a charge-coupled device (CCD) camera of
various types for visible object scenes or an infrared camera for infrared
object scenes. The output of the camera 11 is coupled to a spatial light
phase modulator (SPM) 13, to be discussed hereinafter, through a
sample-and-hold circuit 15. The SPM 13 is placed in one of two optical
paths of a phase conjugate interferometer 14. The interferometer 14
includes a phase conjugate mirror comprising a fast photorefractive
semiconductor crystal 43, such as gallium arsenide (GaAs). Instead of the
GaAs crystal, one could use other photorefractive semiconductor crystals,
such as InP, GaP, CdTe, or any suitable fast photorefractive material.
The phase conjugate interferometer 14 includes first, second, and third
beam splitters 21, 23, 25; a high speed movie camera 27; first, second,
third, fourth, fifth, sixth, and seventh mirrors 29, 31, 33, 35, 37, 39,
41; the fast photorefractive semiconductor crystal 43, and a laser source
45 that provides a coherent light beam of wavelength compatible with the
operation of the photorefractive crystal 43.
The high speed frame camera 27 may be an infrared vidicon camera, an
infrared semiconductor detector array camera, a charge coupled device
(CCD) camera, or a charge transport device (CTD) camera, for example, as
long as its operational wavelength is compatible with the laser
wavelength. The laser source 45 provides a coherent light beam .lambda.
that may be a 1.06-micron Nd:YAG laser, a tunable Ti-sapphire laser, or a
semiconductor injection laser, for example.
The light beam .lambda., emitted from the laser source 45, is split into
three beams BEAM1, PUMP1, and PUMP2 by two beam splitters 23, 25. BEAM1 is
then split into two source beams S1, S2 by a beam splitter 21. The source
beam S1 passes from the beam splitter 21 through the SPM 13, during which
the phase front of the source beam S1 is spatially modulated in accordance
with the image signal from the camera 11. The source beam S1 and source
beam S2 are both incident upon a polished surface of the GaAs crystal 43,
after reflection at mirrors 29 and 33, respectively.
Each source beam S1, S2 creates a respective index grating with a coherent
beam, PUMP1, from the laser source 45. The gratings are generally
overlapping and have a slightly different orientation with respect to one
another. Another beam, PUMP2, also from the laser source 45, but not
necessarily coherent with respect to the other beam PUMP1, travels in the
opposite direction from PUMP1 and enters the crystal 43 at an opposite
polished surface. Parts of this beam PUMP2 are diffracted by the gratings
of the crystal 34, forming two phase conjugate beams, PC1 and PC2. As
shown in FIG. 4, the two phase conjugate beams PC1, PC2 travel along the
same optical paths as beams S1 and S2, but in a direction opposite
thereto. These two beams PC1, PC2 combine at the beam splitter 21 and form
an output beam OUT1, which is imaged on the camera 27.
Because of the reflection of the beam PC2 at the beam splitter 21, the
reflected beam of PC2 has a one hundred eighty (180) degree phase
retardation with respect to the transmitted beam of PC1. After passing
through the beam splitter 21, the two beams PC1, PC2 create a destructive
interference image at the camera 27. By properly adjusting the relative
intensities of the two source beams S1 and S2, a complete destructive
interference can occur due to the nature of phase conjugation, namely,
there is no output light. There are several well-known techniques for
adjusting the relative intensities. A simple one is to insert a variable
neutral density filter (not shown) in the path of the beam S2.
In operation of the overall system of FIG. 1, an incoming object scene 51
with moving objects is picked up by the TV camera 11, and the time
variation of the scene is sequentially sent to the SPM 13 placed in the
optical path S1 of the phase conjugate interferometer 14. If there is no
moving object in the scene 51, the interferometer 14 is in balance and
there is no output signal detected by the camera 27 in the interferometer
14. If there are several moving objects, images of those with speeds
larger than the MDS of the system appear in an output 57. Objects with
speeds less than the MDS value are not detectable. As illustrated in FIG.
1, the incoming object scene 51 contains a fast-moving plane 53 and a
slow-moving tank 55. The plane 53 is observed and tracked by the system,
but the slow-moving tank 55 is not clearly seen in an output 57.
The spatial phase modulator 13 is a device which can change the refraction
index of its active material in response to an external signal, such as an
applied electric field, equivalent to a change in optical path length
through the device. As shown in FIG. 2, the SPM 13 may comprise a matrix
of individually addressed transparent electrodes 71 on the top surface
thereof, and a planar ground electrode 73 on the bottom surface 74. Each
individual transparent electrode 71 can have a separate voltage level
applied thereto via a respective contact 77. The contacts 77 are shown
schematically for purposes of clarity. Sandwiched between the electrodes
71, 73 is a layer 81 of electro-optical material having a predetermined
electrical orientation. This layer 81 of electro-optical material is made
of common electro-optic materials, such as liquid crystals, lead lantalum
zirconate titanate (PLZT), lead zirconate titanate (PZT), or others. The
SPM 13 has a width "w" which is equal to the length of one wavelength of
the coherent beam .lambda..
The SPM 13 of FIG. 2 is used in conjunction with an electronic sample hold
circuit 15 which is of a frame storage type, e.g., a frame memory, for
storing an electrical signal for each element or pixel of an array
corresponding to the image of the object scene 51, as known in the art. A
voltage corresponding to the stored electrical signal of each pixel is
applied to a respective corresponding electrode 77 of the SPM 13. For
example, if the pixel values represent a logical "1," a voltage will be
applied to an electrode 71 as shown in FIG. 3; whereas, if a signal
corresponding to a logical zero is stored at a corresponding pixel in the
frame storage, no voltage will be applied to the corresponding electrode,
as shown in FIG. 4.
The SPM 13 operates as follows. The light beam passing through the portion
of the SP 13 having the applied electrical field will interact with the
electrical field, causing an electro-optical effect. This electro-optical
effect increases the refraction index of the light beam passing
therethrough, thus decreasing the velocity of the light beam, in
comparison to the light beam passing through the portion of the SPM 13
without the electrical field. Thus, the light beam that has passed through
the electrical field is now behind the other light beam, is 180 degrees
out of phase therewith, and has thus been phase modulated. A light beam
which is modulated by the SPM 13 is modulated in a spatial direction,
transverse to the direction of propagation of the beam. The amplitude of
modulation is proportional to the applied voltage.
In order to demonstrate experimentally the motion detection capability of
the fast photorefractive semiconductor 43, a moving mirror, such as a
mirror mounted on a piezoelectric transducer, may be used as a one-pixel
spatial phase modulator. According to current camera technology, signals
from the camera 11 are electrical and in sequence. Therefore, it is
desirable that the spatial phase modulator used in the subject motion
detection and tracking system be electrically addressable and pixelized if
it is to employ conventional camera technology.
Typical results of an experiment using a GaAs phase conjugate
interferometer 14 with a moving mirror as just described are shown in a
pair of graphs 60, 62 of FIG. 5, where time is along the abscissa and
voltage is along the ordinate. A slow motion graph 62 shows that there is
no output signal when the mirror is steady or moving slowly. A fast motion
graph 60 shows a transient spike-shape output signal 61 appears when the
mirror moves fast. The signal lasts considerably longer than the motion,
because the time required for building a new index grating to adapt the
change is longer. As discussed hereinafter, the slow-moving object can
also be detected if a proper sample-and-hold circuit 15 is implemented in
the system. The grating formation time is reciprocally proportional to the
square of the grating periodicity and to beam intensity. This provides a
wide range of minimum detectable rates. For example, the response time in
GaAs can vary from a fraction of a second to tens of microseconds.
FIG. 6 illustrates expected observations of motion detection and tracking
using the preferred system 10. Attention is paid to observation of the
motion of a small portion 70 of an airplane wing 74. Two sequential
pictures 76, 78 of the portion 70 of the wing edge 74 are shown at a
leftmost portion of FIG. 6. The airplane 53 is assumed to be moving from
right to left in the figure. The system output 57 is the difference
between the two pictures 76, 78 and lasts approximately the time similar
to the grating formation time, T.sub.gf. If the time required for the wing
edge 74 to move from one pixel 80 to another pixel 80, T.sub.p, is
approximately equal to T.sub.gf, a sharp picture of the airplane may be
observed as illustrated in the middle picture 82 in rightmost portion of
FIG. 3. If T.sub.p is longer than T.sub.gf, a faint signal can be
expected, as seen in a top most picture 84. If T.sub.p is larger than
T.sub.gf, an elongated picture 86, that is, elongated along the motion
direction, may be observed. These expected phenomena can be used to
determine the speed of the moving object.
In addition, implementation of an appropriate sample-and-hold circuit 15
between the scene input camera 11 and the SPM 13 can enable the system to
detect and track slow-moving objects. The sample-and-hold circuit 15
retains an input signal received from the camera 11. The retained signal
is held for a predetermined amount of time, before the next input signal
is received by the sample-and-hold circuit 15. Thus, the frames output by
the camera 11 may be delayed so that the difference in motion of objects
imaged by the camera 11 is larger. This process enables a lower MDS.
The GaAs system just disclosed has a relatively high degree of immunity to
low frequency mechanical vibration and air turbulence. This is due to the
fact that the response of GaAs is fast and the formation of the grating
can follow the disturbance. This is an important advantage in practical
application.
Those skilled in the art will appreciate that various adaptations and
modifications of the just-described preferred embodiment can be configured
without departing from the scope and spirit of the invention. Therefore,
it is to be understood that, within the scope of the appended claims, the
invention may be practiced other than as specifically described herein.
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